This invention relates in general to electrostatography and more specifically to a novel system for controlling the charging potential of an electrostatographic imaging member.
In the art of electrostatography an electrostatic latent image is formed on an imaging surface of an insulating layer supported on a conductive substrate. The electrostatic latent image may be formed directly by various well known techniques such as charged stylus writing, corona charging through a mask, shaped electrodes, TESI, and the like. The electrostatic latent image may also be formed by electrophotographic techniques including uniformly depositing an electrostatic charge on a photoconductive insulating layer and exposing the photoconductive insulating layer to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. The electrostatic latent image may be developed to form a visible image by depositing finely divided electroscopic toner particles on the imaging surface. The resulting visible toner image can be transferred to a receiving member such as paper. This imaging process may be repeated many times with, for example, reusable photoconductive insulating layers.
Generally, in electrophotographic imaging systems, the electrophotographic imaging member and developer subsystems are optimized for the specific materials utilized, e.g. photosensitivity, development zone density and the like, and also for copy quality requirements, e.g. low background deposits, low density line reproduction and the like. After optimization, the photoreceptor and developer subsystems are fixed and only minor modifications can be made in charging or exposure levels to meet specific user requirements. In most cases, the only immediate controls over tone reproduction afforded the user are copy lighter control buttons and copy darker control buttons which vary the bias voltage of a development electrode such as the conductive member of a magnetic brush applicator roll. In some electrophotographic imaging systems, the bias to the development electrode is fixed at a given value and the operator is only allowed to vary the exposure. These approaches result in essentially the same, very limited, control over copy quality. A disadvantage of this is that copy darker control buttons enhance low density images, but increase copy background, whereas copy lighter control buttons suppress image information as well as reduce solid area contrast.
Features which allow variable control in a machine of the charge deposited by a corotron, require the use of complex and costly control circuits. The increased costs and complexity as well as the additional space required for control circuits render these approaches undesirable in compact, low volume copiers and printers.